--- Don Weiss <[EMAIL PROTECTED]> wrote:
<snip>
> Can somebody tell me if the following conjecture is
> correct?
>
> The metering system has to know what the metering
> (also of course the
> viewing) aperture of the lens is, right? When you
> mount an EF lens, the lens tells the camera
> during viewing what the metering/viewing aperture
> is; viz., the lens' maximum aperture. The camera
> knows what aperture the lens should stop down to
> during the actual exposure; either by calculating it
> or by being forced by the user via Av mode.
That's right... that's why the upgrade from FL to FD
lenses on the old MF Canon SLR's was such a huge
advance, you could now do coupled metering at full
aperture WITHOUT having to tell the camera what the
maximum aperture of the lens was. The Canon FT *had*
to use stop-down metering, because the meter was not
aware of what the maximum aperture of the lens was.
With the FTB and FD lenses, you could have coupled
metering at full aperture. (This is also why the Nikon
AI lenses were such an advance over the pre-AI ones,
you no longer had to do the semi-automatic "Nikon
twist" to index the max aperture. As far as I can see,
Nikon copied their coupling system from Minolta's MD
mount!)
> Now, when you use Av mode on a fully manual lens,
> where there is no communication of aperture (or
> anything else) betweeen camera and lens. Does the
> camera take note of this fact and then assume that
> the set Av value is the viewing/metering
> aperture?
That is exactly how it works. If it doesn't hear
anything from the lens (ie, you've mounted a manual
lens via an adapter or a T-mount lens that has no
electronic provision to report max aperture to the
camera) it assumes you're already at the taking
aperture; notice that this works in both P (program)
as well as Av (aperture value) modes!
> I don't see any other assumption the
> camera's designers could make that would allow any
> exposure automation (in ETTL mode)
> possible with a manual aperture lens.
Look at any of the built-in coupled metering systems
that did not mechanically report max aperture in the
"old days" (particularly Pentax screw mount cameras),
you must use stop-down metering for any autodiaphragm
lens.
The off the wall exception is the Miranda Sensorex and
Sensorex II... both could perform coupled metering at
full aperture, but you must set a knob on the camera
to tell the meter what the max aperture of the lens is
every time you change lenses... sort of like the
system that was used on the Nikons and Nikkormats
before the FTN Photomic and N-mat FT-N.
I shoot often with an old Exata VX-IIa... talk about a
lesson in technology appreciation... it doesn't have a
built-in meter, instant return mirror, autodiaphragm,
or an eyelevel prism. It's a breeze shooting EOS after
that thing. :-)
MadMat
__________________________________________________
Do You Yahoo!?
Get personalized email addresses from Yahoo! Mail - only $35
a year! http://personal.mail.yahoo.com/
*
****
*******
***********************************************************
* For list instructions, including unsubscribe, see:
* http://www.a1.nl/phomepag/markerink/eos_list.htm
***********************************************************
